Conventionally, the following method has widely been used as a general method for treating an organic waste water. First, organic components of an organic sludge are biologically digested by aerobic or anaerobic microorganism decomposition, such as an aerobic digestion method, an anaerobic methane fermentation method, and the like, such that organic matter is decomposed into gas components, such as carbon dioxide, methane gas, and the like. Then, the sludge containing mainly a microorganism biomass generated by the biological digestion and untreated residual solids is solid-liquid separated by means of a precipitation vessel, or other suitable apparatus. While the treated, supernatant water is suitably processed, the excess sludge usually is discarded by dumping at sea or on land. However, dumping excess sludge at sea adversely affects the environment. Therefore, dumping a sludge at sea has been essentially prohibited because of global environmental protection initiatives. Furthermore, each year it has become harder to find a land disposal area in which an excess sludge can be dumped.
The present invention is related to an activated sludge treatment method and apparatus capable of substantially reducing the amount of excess sludge generated by treating organic waste water. It has been reported previously that an activated sludge treatment method and apparatus is characterized in that, as shown in FIG. 4, an organic waste water introduced from an organic waste water reservoir 1 is subjected to aerobic biological treatment in an aeration tank 2, the treated waste water then is solid-liquid separated into treated water and a sludge by a precipitation device 3. A portion of the sludge separated by precipitation device 3 is returned to aeration tank 2 through a recycle route 4, an excess sludge separated by precipitation device 3 is heat exchanged by a heat exchanger 5, and then is solubilized by a solubilizing treatment device 6. A treated solution obtained by solubilization performed by solubilizing treatment device 6 is returned to aeration tank 2 through a return route 7 (see Japanese Patent Application No. Hei 7-163355).
In one embodiment of the invention disclosed in Hei 7-163355, about 50% of a solid is solubilized and about 50% of a volatile matter is removed, when the treatment of the excess sludge is performed over a residence time set at 5 days. If the excess sludge is to be fully solubilized, the treatment then is performed at a high temperature. Accordingly, a large amount of excess sludge is treated for a long time by means of a solubilizing treatment device including a large capacity vessel. Consequently, the scale of the required equipment is large, which results in an increase in operating and maintenance costs for heating and the like. Thus, there are disadvantages to industrially performing the method set forth in Hei 7-163355.
Japanese Patent Application No. Hei 7-260287 discloses a sludge digesting treatment method comprising a step of producing enzymes capable of solubilizing sludge and an apparatus for performing the same (see FIG. 5). The enzyme producing step is performed by incubating a microorganism obtained from a sludge, or a different source, under proper conditions, and the enzyme produced and secreted by incubation is used to promote solubilization of the sludge in a solubilizing vessel, thereby reducing the amount of excess sludge. Although the specification of Hei 7-260287 discloses preferred conditions, such as temperature, pH, time, and the like, under which the solubilizing reaction is conducted, it is apparent that such conditions are appropriately selected depending on the concentration of sludge to be treated, the composition and physical properties of the organic matter or the like, and the enzymatic properties of the secreted enzyme. The selection criteria, however, have not been clearly established. According to this method, it is expected that the solubilization efficiency in the solubilizing vessel can be increased. However, because the steps of producing and digesting the enzyme are essential, the entire operation is relatively complicated, which often results in an increased size of equipment.